Three-phase electric power is a common method of electric power transmission and is implemented by three conductors each carrying voltage waveforms 2π/3 radians (120° or ⅓ of a cycle) offset in time. Public power facilities that deliver electric power to domestic, industrial, and commercial buildings in most nations including, e.g. the United States and much of Europe, generate three-phase power. Although voltage produced by these power facilities typically varies throughout the world, e.g. 460V 60 Hz in the United States versus 230V 50 Hz in much of Europe, the 120° phase separation of a three-phase system is always approximately constant, as it is a defining characteristic of three-phase power.
Three-phase electric power is appropriate for various forms of electric equipment including e.g., motor drives, appliances, boilers, space heaters, electric arc furnaces, rail cars, air conditioning units etc. Typically, these applications are designed around a balanced three-phase power input. Balanced power is a result of all three phases having a substantially identical voltage and a 120° shift in phase with respect to each other. Balanced three-phase power has several distinguishing characteristics. First, balanced power provides constant, non-time varying electric power transfer. Constant power transfer is a desirable condition, as large motor drives and generators will run much more smoothly on constant power than on varying input power. Second, balanced power systems have zero neutral current. This ensures a more efficient delivery of electric power. To take advantage of these balanced phase characteristics, most three-phase load devices are designed to run on balanced three-phase power.
AC variable frequency motor drives are a good illustration of a common device that derives power from three-phase electric power input. Such motor drives are robust pieces of equipment used in commercial and industrial facilities capable of years of dependable service. More specifically, simple design, inherently high starting torque, and high efficiency are common characteristics. Typical applications include driving machinery, e.g. industrial pumps, fans, blowers, compressors, conveyors belts, etc. In addition, three-phase motor drives are more compact, require less current and consequently can produce less heat, are less expensive, and vibrate less than comparable single-phase counterparts. Three-phase motor drives are so desirable for medium to high power applications of AC/DC conversion, that single-phase counterparts above 10 horsepower (HP) are exceedingly uncommon. So long as a three-phase motor drive is properly sized, well maintained, and supplied with balanced three-phase power, such a motor drive constitutes one of the most efficient methods for powering contemporary machinery.
As mentioned previously, three-phase power devices typically operate on balanced power. If a phase shift or voltage associated with one of these three phases deviates too much, the three-phase power can become imbalanced and be detrimental to a device. Voltage imbalances can occur on a utility provider's equipment or on a consumer's electric load devices if, for example, too many single-phase loads are added to one or more phases of a three-phase signal. In contrast, a complete loss of one of the phases is the worst case of voltage imbalance. A cause of phase loss can include e.g., a downed power line, a blown fuse on a transformer associated with a phase, a single-phase overload condition, an open winding of a supply transformer providing a phase, a lightning strike, and/or equipment failure associated with one or more of the three phase input lines, etc. Running a device under single-phase conditions for higher than rated loads can result in thermal overload of the device.
As an example, in the case of a three phase AC to DC applications, e.g. AC to DC motor drives and the like, if a device is subject to a loss of phase while running, also known as single-phase operation, under higher than rated load the remaining two phases will continue to power the device (e.g. attempting to supply torque and power required by the motor load). Unfortunately, phase loss will result in an unbalanced input signal. AC to DC power devices (requiring balanced input power) operating under these conditions can overheat, induce transformer failure, etc., if the phase loss condition persists. Therefore phase loss is one of the most common catastrophic failure mechanisms for electronic components and devices running at relatively high power loads.
Complicating this problem in the case of the three-phase motor drive is the fact that these motor drives are typically very dependable; they are often installed only with minimum protection as required by the National Electric Code. The National Electric Code requires overload protection on each input power phase, e.g. fuses or thermal overloads (heaters), sized to prevent the motor drive from drawing too much current, but such protection cannot preclude damage that results from severe voltage imbalance or complete loss of a phase. Therefore, specific measures to obviate phase loss and other severe voltage imbalance can be necessary.
Traditional strategies to protect against single-phase operation include voltage relays or time-delay fuses matched to measure motor drive loading. These strategies proved insufficient, as phase loss still occurred under some conditions. Solid-state phase monitoring devices were developed to provide much more robust protection. Such solid-state phase monitoring devices are typically hardware devices that can be coupled with peripheral components, e.g. a motor drive starter, or can be stand-alone devices. For example, a motor drive starter can be specified with a solid-state overload block instead of a thermal overload. The cost of the solid-state option is comparable to a standard thermal overload. Stand-alone phase monitors are essentially solid-state voltage relays that can sense voltage imbalance. They have output contacts that can be tied into three-phase motor drive control circuitry to take it off line (shut it down, so that it no longer draws power) if a severe voltage imbalance or loss of phase occurs.